Light emitting elements, such as light emitting diodes (LEDs), enjoy increasing use in a wide range of applications, some examples being back light sources in liquid crystal displays, flashes for charge coupled device cameras, general lighting, as well as other applications. In many of these applications, LEDs of different colors are used, e.g. arranged in an array, to produce various color points. The operating conditions for the LED array may be equally as diverse as the array's application, such operating conditions requiring, e.g., low power, high operating temperature, and fast LED activation and deactivation times.
Typically, each LED array is powered by a driver circuit operable in one of several different driving modes depending upon the desired light effect. The LED driver circuit may be driven in a constant current mode, whereby the LED array is supplied a constant current to provide light at a constant intensity. The LED driver may also operate in a variable current mode, whereby the LED array is supplied a varying current to produce a varying intensity of light. The LED driver may also operate in a pulse width modulation (PWM) mode, whereby the LED array is supplied using a PWM current waveform in which the on period of the PWM waveform determines the time period over which the LED array is activated, and thus determines the light output and thus the color point of the LED array. The PWM mode may be implemented with either the constant current mode or the varying current mode to provide a combination of each of these attributes, i.e., constant or varying light intensity.
Unfortunately, a large number of circuit components are needed to provide the aforementioned functionality. For example, when a constant current, PWM mode of operation is desired, at least one current source for the LED array and one switch for each LED in the array is typically required. In the case in which a varying current mode of operation is desired, a complex current source operable to quickly change current levels is required. In the case in which a varying current, PWM mode of operation is desired, a complex current source and one switch per LED within the array is usually required.
A high part count for operation and control of the LED array degrades LED performance in a number of ways, each component increasing power consumption of the LED array and contributing parasitic effects which operate to reduce activation and deactivation times of the LEDs. Furthermore, when the LED array is implemented in a high temperature application, each component will require a high temperature rating, a capability that further increases the cost for each required component. Acknowledgement of the problems associated with high part count LED drivers can be seen in U.S. Pat. No. 5,736,881 to Ortiz disclosing an PWM LED driver and LED array configuration in which one current source is used to control multiple LED strings.